I cannot imagine 10^30 kg of H2O floating around in space but if it did what would be the effect of that vast amount of Oxygen on the start up of nuclear fusion that is required for a star to get going presumably the Oxygen would accumulate at the centre with an outer shell of Hydrogen but I doubt if pressures would be high enough for fusion to start up.

I cannot imagine 10^30 kg of H2O floating around in space but if it did what would be the effect of that vast amount of Oxygen on the start up of nuclear fusion that is required for a star to get going presumably the Oxygen would accumulate at the centre with an outer shell of Hydrogen but I doubt if pressures would be high enough for fusion to start up.

Thank you syhprum. Don't worry..I'll imagine it for you... [].............I guess I was just getting all hypothetical...though, one might think that with enough space and time many a strange event occurs. I appreciate your postulation.

For a ball of water the mass of the Sun, the extreme temperatures in the core would break down the water into oxygen and hydrogen plasma (as stated before). It would be too massive to be fully convective, so oxygen should accumulate in the core and hydrogen in an envelope around the oxygen. However, while the hydrogen and oxygen plasmas are still mixed, the hydrogen might be able to undergo fusion for a while (at least until the hydrogen migrates into the upper layers of the star). Once the hydrogen leaves the core, I think the star would be extinguished: the mass of this "star" would be insufficient to start oxygen fusion. The minimum mass required seems to be 8-9 solar masses. If it was of that mass, its oxygen would be used up in a maximum of five years, the time span becoming shorter as the mass increases (thus increasing core temperature and reaction rate).

Then again, the temperatures deep in the hydrogen envelope might still be enough to keep fusion going. I'm not sure. I guess I'd need to research more.

There would likely be several phases of water (known and unknown) in such a scenario. Because the formation of such a body would likely involve a significant mass of water to "falling together," causing the temperature to rise substantially. But the pressures will also be extraordinarily high. There might be some unusual types of ices formed, if the thing doesn't heat all the way up to plasma and start fusing (or crush itself all the way to a black hole--I don't know how much water would be in a "star sized" body)

I would imagine that a lot of the water would be in some type of supercritical phase, as temperatures are likely to be above 374 °C, and pressures would certainly exceed 22.064 MPa (3200 psia or 218 atm).

If the temperature exceeds 1100 °C the water near the surface (lower pressure) might spontaneously break apart into H2 and O2, and the star could phase separate into the elements.

It's an interesting question.I guess it depends how we make this "star sized blob of water.To make it easy, lets imagine that we start with a Earth sized blob.It's not going to do any fusion or anything else that makes heat , so eventually it will cool down- some will evaporate and we will end up with a big lump of ice.Now, imagine that , from time to time, someone turns up and drops a big bucket of water onto it.Then they go away and leave it to cool down again. Then they return with another bucketful.As long as they do it slowly enough there's no reason for it to heat significantly.From time to time you might get "blob-quakes as the inner layers are compressed into denser forms of ice.

If you made a star sized block of ice slowly enough I'm not sure it would do anything.Eventually you would get enough ice to collapse into a neutron star

Not sure how slow this would need to be- possibly longer than the age of the universe.

Molecular gas clouds start with temperatures near absolute zero - around 10K, and not far above the Cosmic Microwave Background Radiation (2.7K). So a hypothetical water star would start as ice.

Once a molecular cloud has sufficient mass (a thousand to a million times the mass of the Sun), higher-density internal clumps can contract under their own gravity to form stars, planets, comets and asteroids. This is called a stellar nursery. Once the young stars start fusion, the temperature rises and the dust clouds dissipate.

The temperature has to be so low, otherwise the thermal motion of its atoms would cause the particles to drift away from each other before it could form particles as large as smoke particles.

In today's universe, the composition of the gas clouds is around 70% Hydrogen (left over from the big bang) and most of the rest is Helium (enriched beyond the 10% which formed in the big bang, by stars that have subsequently gone supernova). There are other impurities in these clouds, like carbon and oxygen, which were spewed into space when heavier stars went supernova; but the concentration is very low (<1%). When radio astronomers search for these gas clouds, they typically search for lines associated with molecules like carbon monoxide (CO).See: https://en.wikipedia.org/wiki/Star_formation#Interstellar_clouds

Why Water Stars Don't FormThe composition of these clouds is mostly hydrogen & helium. If you put a large amount of these together, you will get hydrogen fusion - a bright star; there is not enough oxygen to form a water star.

However, water has a very high melting point (roughly 273K), so you could imagine that as the molecular cloud near a new star heats up, any wisps of hydrogen, helium, oxygen and nitrogen will be evaporated and driven away by the stellar wind of the young star. However, high melting-point molecules like water, CO2, graphite and NH3 could clump together to form comets - a "dirty snowball".

Diamond Stars?If you want to get romantic, it is thought that stars a bit more massive than the Sun could burn most of their Hydrogen & Helium to Carbon, but not have enough mass to carry fusion any further (ie not produce much oxygen).

Huh? Never knew that comets are made of water. I thought comets are hot since I think of them as shooting stars, or are those water boiling?

Shooting stars are actually meteors that give off light as they burn up in the atmosphere. Even though you can see their light from the ground, they can be very tiny (the size of sand grain, for example).